1. Field
This patent specification relates to a system and a method of gradation conversion of multi-valued data, capable of setting conversion characteristics with more ease even for data having mixed image attributes and reducing the amount of data and the size of memory required, and image processing and image forming apparatus incorporating the conversion system.
2. Discussion of the Background
The gradation conversion described herein is in use for read gamma conversion, print gamma conversion and other similar conversions.
The read gamma conversion is utilized for correcting, and changing to desired image representation, the image data acquired by a document scanner or digital camera with respect to distortion caused by, for example, imaging characteristics of imaging device, or of changing the image data.
In addition, the print gamma conversion is used for altering the brightness, contrast or color of CG (computer graphics) images, and/or of changing image data or CG data to be the image data which are compatible with the representation characteristics of a printer and suitable for output record.
The above noted examples are intended to be illustrative but not limiting the scope of the present disclosure.
Several methods have been disclosed previously on the gamma-correction, in which output densities corresponding to input densities are once stored in an external memory for the entire range of density and which an output density of current interest is obtained by reading out the density at the corresponding input density address among the stored densities.
In these methods, however, there encountered several drawbacks such as a large amount of memory data required and insufficient flexibility for adopting the change of gamma curve.
The use of polygonal lines to approximate the density curve is disclosed in Japanese Laid-Open Patent Application No. 8-18826, in which a digital gamma conversion circuit is proposed to perform gamma conversion for correcting video signals to adapt display characteristics (gamma curve) of display device utilizing the polygonal line in place of LUT (Look Up Table) and obtain corrected image data after the computation with the polygonal line, so that gamma characteristics can be altered in real time.
Another method of gamma conversion is disclosed in Japanese Laid-Open Patent Application No. 10-145806. In this method, the change or modification of the gamma conversion is carried out by storing plural sets of gamma conversion data in LUT, generating set(s) of gamma conversion data that are different from the plural sets in LUT, writing the set(s) into RAM and utilizing the RAM for the conversion.
Furthermore, a gamma conversion circuit is disclosed in Japanese Laid-Open Patent Application No. 2000-184236, in which gamma conversion is performed by approximating gamma conversion characteristics to a polygonal line and performing the gamma conversion on input video data V0 using a linear expression, Vc=a·V0+b.
Although several conversions on data can be cited in general such as correction of RGB image data (scanner and read gamma corrections), color conversion from RGB to YMCK, correction of YNCK image data (for example, printer gamma conversion) and so on, it is preferable for the conversion to be provided with the capability of such as, for example, switching or adjusting its correction and conversion characteristics.
When one process mode is designed to be selected from plural process modes to satisfy the requirement from the conversion characteristics, processing data of these plural process modes have to be prepared, so that the amount of data to be processed further increases.
For example, three LUTs are utilized, i.e., one for each of RGB colors, in the case of the read gamma conversion. And, since four LUTs for YMCK colors are similarly used for the printer gamma conversion, the amount of data in the LUTs amounts to that multiplied by ‘4×a’, if a plurality, ‘a’ kinds, of LUTs are prepared.
In order to reduce the amount of data and the size of memory to be prepared for gamma conversion, and further to facilitate the change of gamma conversion characteristics, the present inventor has disclosed (Japanese Laid-Open Patent Application No. 2000-83171) a system for performing gamma conversion simultaneously on a large number of image data on one single raster through the computation adopting a set of linear approximation expressions using SIMD (Single Instruction stream Multi-Data stream) processor.
In the case when specified data conversion characteristics are converted according to several data conversion characteristics prepared beforehand, it is preferable to affix attribute data to the data to be converted and select suitable data conversion characteristics corresponding to the attribute data.
For example, in the gamma conversion of the image data acquired by a document scanner (scanner gamma conversion) and the print gamma conversion for fitting for representation characteristics of the printer on which the image data are to be printed out, gamma conversion characteristics may be selected preferably based on the attributes of either character and line drawing (binary image) or photograph (mesh dot image).
That is, for the binary image, gamma conversion characteristics capable of enhancing the contrast of images are selected corresponding to the attribute for binary image. For the mesh dot image, in contrast, gamma conversion characteristics capable of smoothing the density change are selected corresponding to the attribute for mesh dot image.
When plural kinds of LUTs in use for the gamma conversion are so prepared in the memory area so as to satisfy the requirement from the conversion-characteristics, the size required in memory has to become enormous.
It is highly preferable, therefore, to reduce the amount of required data by carrying out the conversion method for simultaneously gamma converting the large number of image data on one single raster through the computation adopting a set of linear approximation expressions using SIMD processors.